1. Field of the Invention
The present invention relates to a novel epoxy resin adhesive composition. More particularly, it relates to an epoxy resin adhesive composition comprising a powder core/shell polymer in which the core and the shell comprise an acrylate polymer or a methacrylate polymer and having excellent impact resistance and semi-gelling property simultaneously.
2. Description of the Prior Art
Epoxy resins have various excellent properties and have been widely utilized in various applications such as adhesives, adhesive films, bulk resins, resins for casting, resins for powder molding, coating materials, sealing materials for electronic circuits, base materials for various composites and the like other applications.
However, epoxy resins have a problem that impact resistance is insufficient and naturally various improvements have been made to solve the problem. The methods of improving the impact resistance of epoxy resins can be divided into two major kinds, which are improvement by modifying chemical structure of the epoxy resin itself and improvement by addition of an impact resistance modifier prepared separately to the epoxy resin. Methods of the former kind alone cannot provide an epoxy resin having satisfactory impact resistance.
As methods of the latter kind, the following methods have been known: (1) addition of a soluble elastomer monomer to the epoxy resin before curing, followed by polymerization of both components; (2) addition of a compatible elastomer polymer to the epoxy resin before curing; and (3) dispersion of fine particles of an impact resistance modifier polymer in the epoxy resin before curing. As an example of the method (1), a method of forming rubber domains of 0.1 to 0.2 .mu.m in an epoxy resin by forming SIPN (Simultaneous interpenetrating networks) of n-butyl acrylate in the resin was reported (Journal of Polymer Science, Symposium, volume 46, pages 175.about.190 (1974). This method has the problems that softening point of the resin is generally decreased and that consistent mechanical strength cannot be obtained. As the method (2), various methods have been known about rubber modification of the resin by addition of an elastomer component such as butadiene acrylonitrile copolymer rubber having a carboxyl group and an amino group as the terminal groups of the molecule. Some of these methods have been practically applied but the adhesives prepared by these methods do not have satisfactory impact resistance or toughness for use as a structural adhesive. As the method (3), various kinds of impact resistance modifier have been proposed but none of them have sufficient semi-gelling property.
It is generally known that addition of a rubber component having a glass transition temperature of -30.degree. C. or lower as an impact resistance modifier of a plastic improves the impact resistance of the plastic remarkably by absorption of external stress by the modifier. However, when many of the rubber components of this kind are mixed with a matrix of an epoxy resin in a liquid state, the dispersion tends to be affected by the conditions of mixing and viscosity of the composition prepared is unstable during storage. Thus, this kind of material is not practical for use as an adhesive for automobiles to which storage stability for a long time is required.
Semi-gelling property is another important property for epoxy resin adhesives. It is known that a core/shell type modifier comprising (meth)acrylate polymer is effective for providing an epoxy resin composition with the semi-gelling property (Japanese Laid Open Patent Application Heisei 2-80483). The semi-gelling property is the property whereby an adhesive in a liquid form or a paste form solidifies to a non-adhesive or adhesive solid form by heating at a temperature lower than the heat curing temperature and, in this form, the adhesive is still in a fragile condition because the epoxy resin component is not hardened yet. The semi-gelling property has the following advantage. In the automobile industry, a metal substrate is sometimes subject to workings such as folding, cutting, degreasing and acid treatment after it has been applied with a heat curing adhesive composition based on an epoxy resin. During the workings, problems are likely to arise that adhesives may be separated from the substrate or scattered, that excess adhesive material sticking out of the part of adhesion must be removed to deteriorate the working environment and that a pretreatment solution of coating is contaminated by elution of the adhesive material. The semi-gelling of the adhesive composition by heating for a short time after application of the adhesive composition on the substrate makes removal of the excess adhesive material from the substrate easier and the problem of the contamination of the pretreatment solution of coating can be solved at the same time.